Prior racket stringing machines generally are operated by experience without any information about the tension of the string to be tightened. The prior art disclosed by Franklin W. Held provides a device which can be stopped automatically when reaching a predetermined string tension during stringing, but the device is manually operated. U.S. Pat. No. 4,546,977 discloses a clamping device with fast locking ability, but it is suitable for only longitudinal clamping, lateral clamping is still to be developed.